The present invention relates to methods and apparatus for performing dot matrix printing and more particularly to methods and apparatus for computerized printing using prestored character data having reduced memory storage capacity.
Dot matrix methods for printing characters, numerals, and the like are advantageous, compared with printing methods using font, that is, individual pre-set character elements, in that the character patterns printed can be assembled using software, that is, programmed operations on stored data. Moreover, the printing of enlarged, reduced or italicized characters can be easily formed. Accordingly, dot matrix methods are quite flexible and have found wide application. On the other hand, since characters are formed with a limited number of dots, curved or oblique lines are apt to be distorted thereby reducing the character resolution. In order to eliminate this problem, it has been the practice to proportionally increase the number of dots and hence the capacity of memories used to store the character patterns.
Many methods to restrict this increase in the number of dots and related memory size have been proposed. These methods can be classified into two groups. With the methods of the first group, a portion of the dots which make up the characters are generated operationally to thereby avoid a need for a portion of the memory capacity. With the methods of the second group, each character pattern is divided into a plurality of subpatterns and the number of bits in the pattern is determined according to the frequency of identical subpatterns which are used.
Although the first group of methods is advantageous in that the capacity of the memory which stores the character patterns does not increase, the operational process required for implementation of the method is often complicated and it is impossible to always reproduce accurate characters.
As to the second group of methods, there is some advantage as to the needed memory storage brought about by divisions into appropriate subpatterns. However, the savings in memory storage capacity obtainable with the methods of the second group are at most 20-30% and the accompanying process needed for character decoding is complicated and time consuming.
What is needed is a method and apparatus for dot printing which relies on compressed data for an entire character, thereby avoiding both special operations and sub-patterns decoding to print a character having a large number of dots.